The invention relates generally to fluidized bed gasifiers and to the production of gasified fuel via and/or in conjunction with the gasification of solid fuels in a fluidized bed gasifier featuring a rotating auger conveyor. More particularly, it deals with improvements to the gasifier that facilitate the continuous controlled movement and gasification of solid fuels in the combustor/gasifier; and facilitate production and enrichment of gasified fuel via and in conjunction with such a gasifier. These and other improvements taught herein in relation to the operation of a gasifier represent significant advances in technology related to the environmentally sound utilization and processing of solid fuel for the production of energy via such fluidized bed gasifiers.
Much of the world's energy needs have been, and continue to be, filled by hydrocarbon fuels. In the past, such fuels provided a convenient, plentiful, and inexpensive energy source. The current rising costs of such fuels and concerns over the adequacy of their supply in the future has made them a less desirable energy source and has led to an intense investigation of alternative sources of energy. The ideal alternative energy source is a fuel which is renewable, inexpensive, and plentiful, with examples of such fuels being the byproducts of wood, pulp, and paper mills, and household and commercial refuse.
The use of such alternative energy sources is not problem-free, however, since there is reason for concern over the contents of the emissions from the combustion of such fuels as well as the environmental ramifications of acquiring and transporting the fuel and disposing of the residue of combustion. Starved-air gasifiers, wherein the air supplied for combustion is controlled in order to control temperature conditions (and the rates of combustion) so as to gasify the fuel as completely as possible, have proved very useful in the utilization of such alternative energy sources while simultaneously maintaining a high degree of environmental quality in emissions. Such starved-air gasifiers are capable of gasifying various types of fuel and producing significant amounts of syngas and heat which can be employed for any number of purposes including the production of process steam for use in manufacturing and in the generation of electricity.
Unfortunately, most starved-air gasifiers, as originally developed and operated, were not entirely satisfactory in processing the gasifiable elements of the fuel at high throughput while not producing noxious emissions. This problem resulted, in part, from the use of such gasifiers to burn a wide variety of fuels, including many which were non-homogeneous, such as household or commercial refuse. While the pollution problem can be solved to a degree by the utilization of scrubbers and other antipollution devices, such mechanisms are very expensive and their cost may militate against the use of alternative energy sources previously described.
Many of the drawbacks of such prior art devices were overcome by the development of the auger gasifier by the inventor and others. See, U.S. Pat. No. 4,009,667 (describing the original auger gasifier utilized in the system); U.S. Pat. No. 4,315,468 (describing a control means for the system); U.S. Pat. No. 4,331,084 (describing a refuse fuel feed mechanism for the system); U.S. Pat. No. 4,331,085 (describing a flame stabilization means for the system); U.S. Pat. No. 4,332,206 (describing an afterburner for the system); U.S. Pat. No. 4,332,206 (describing a hot gas recycle mechanism for use with the system); and U.S. Pat. No. 6,349,658 (describing an auger gasifier with fluidized bed. The auger gasifier technology taught and described in the foregoing patents offers a cost-effective approach to clean, efficient gasification of prepared solid waste and other solid fuels. It employs a starved-air combustion/gasification technique, ideally utilizing only limited combustion in order to gasify solid fuel in a primary chamber (the “combustor” or “gasifier” chamber).
One of the unique features of the auger gasifier system is its auger. Fuel enters the gasifier at a controlled rate and is shaped into a pile by the first auger flight. It is then pushed and tumbled through the gasifier chamber by the auger. As the auger moves the fuel through this horizontal cylinder, it stirs the material to maximize exposure for oxidation and/or gasification. The pitch of the auger can decrease along the path of material flow to accommodate the decrease of fuel bulk and retention time as the material combusts. (The use of an auger to convey fuel through the gasification cycle results in very accurately controlled movement of fuel through the gasification chamber, in comparison to alternative rotary kiln incinerators). This ability to manage fuel-bed configuration permits control of forced-draft combustion air so as to minimize combustion and gasify nearly all the fuel without complete combustion taking place, thereby allowing the gasifier to operate at what is a uniformly moderate temperature from auto-ignition to desired exit gas temperature.
The combination of fuel bed stirring and air injection with precise temperature control gives the auger gasifier system several advantages over prior technology: Reliability and clean operation; high throughput; low gasifier temperature: longer material life (refractory and auger); fully automatic control; and the ability to combust a wide variety of heterogeneous solid fuels. However, several of these advantages are further strengthened by creation of a fluidized bed via the high pressure input of underfire air into the system via a large plurality of input holes. The fluidized bed can be formed in whole or in part by the addition of a substrate of appropriate granular materials or it can be comprised solely of materials in the gasification process. In either case it acts as an “air bearing” in the auger combustor/gasifier chamber, aiding in the diffusion of gasification air through the material being burned as more fully described in U.S. Pat. No. 6,349,658.
Still, while the auger combustor/gasifier described in the foregoing patents offers a cost-effective approach to clean, efficient gasification, I have found that various innovative improvements, as further taught herein, support and facilitate its operations and efficiency. These improvements include provision of a vertically elongated (“oblong”) primary gasifier chamber with an auger that can move up and down, allowing for large amounts of fuel input when necessary. This improvement, in turn, requires and/or is facilitated by provision for simultaneous elevation adjustments of the auger and bed dam to assure that fuel material is processed in degrees from the top downward without sweeping massive amounts of the fluidized bed materials towards the output end of the chamber. Another improvement involves provision for pressurization of the primary gasifier chamber, allowing substantial improvements in the speed of processing materials through the gasifier. Due to issues arising from thermal expansion of the refractory material lining the chamber, I have also made provision for nozzle and refractory imbedded pipe hole thermal expansion capability. To these I have added provision for steam injection into and/or auxiliary heating of the chamber to enhance gasification and the production of syngas. These changes and improvements serve to create an even more efficient and cost effective system which is well adapted to meet the continuing needs of our modern technological civilization for elimination of waste while producing clean environmentally sound sources of alternative energy therefrom.